Process and apparatus for heating feedwater in a heat recovery steam generator

ABSTRACT

A feedwater heater ( 14 ) in a heat recovery steam generator (A,B) lies within a flow of hot exhaust gas. The feedwater heater ( 14 ) converts subcooled feedwater into saturated feedwater water, the temperature of which is only lightly above the acid dew point temperature of the exhaust gas so that corrosive acids do not condense on coils ( 18 ) of the feedwater heater ( 14 ). Yet the temperature of the saturated feedwater lies significantly below the temperature of the exhaust gas at the coils ( 18 ), so that the coils ( 18 ) operate efficiently and require minimal surface area. Pumps ( 26, 28, 30 ) elevate the pressure of the saturated feedwater and direct it into an economizer ( 64, 90 ) where, owing to the increase in pressure, the water is again subcooled. The economizer ( 64, 90 ) elevates the temperature still further and delivers the higher pressure feedwater to evaporators ( 34, 70, 78 ) that convert it into saturated steam that flows on to the superheaters ( 50, 78, 84 ). Higher pressure pegging stem admitted to the feedwater heater ( 14 ) controls the pressure—and temperature—of saturated steam and water in the feedwater heater ( 14 ).

RELATED APPLICATION

This application derives priority from and otherwise claims the benefit of U.S. provisional application 61/300,222 filed 1 Feb. 2010, which application is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to boilers and more particularly to a heat recovery steam generators having improved feedwater heating.

BACKGROUND ART

Boilers designed to convert liquid water into steam by extracting energy from hot gases have become more efficient over the years, and much of this efficiency derives from extracting heat from the gases at lower temperatures—temperatures at which the gases might otherwise be exhausted to the atmosphere. But this increased efficiency has created its own problems that if left unaddressed can result in corrosion of the low temperature surfaces of the boilers.

Heat recovery steam generators (HRSGs) represent an important class of high efficiency boilers. The typical HRSG operates in a system that includes a gas turbine that drives an electrical generator. The turbine discharges exhaust gas at an elevated temperature, and this gas flows into the HRSG which extracts heat from it to convert subcooled liquid water into superheated steam, usually at several pressures. The steam powers a steam turbine which in turn drives another electrical generator. The HRSG has multiple banks of coils, the last of which in the direction of the gas flow often forms part of a feedwater heater. It receives condensate that is derived from low pressure steam discharged by the steam turbine and elevates the temperature of the water before the water is discharged into one or more evaporators that convert it into saturated steam. Superheaters in turn convert the saturated steam to superheated steam that powers the steam turbine.

The combustion of a fossil fuel, such as natural gas, fuel oil or coal, produces the hot exhaust gas that powers the gas turbine and flow through the HRSG. By the time the hot gas reaches the feedwater heater at the back end of the HRSG its temperature is quite low, but it should not be so low that acids condense on the heating surfaces of the feedwater heater. After all, the combustion produces primary carbon dioxide and water in the vapor phase, but the gas will also include traces of sulfur in the form of sulfur dioxide and trioxide. Those compounds will combine with water to produce sulfuric acid which is highly corrosive. As long as the temperatures of the heating surfaces remain above the acid dew point temperature of the exhaust gas, SO₂ and SO₃ pass through the HSRG without harmful effects. But if any surface drops to a temperature below the acid dew point temperature, sulfuric acid will condense on that surface and corrode it, and the vulnerable surfaces exist on the feedwater heater.

Dew point temperatures vary depending on the fuel that is consumed. For natural gas the temperature of the heating surfaces should not fall below about 140° F. For most fuel oils it should not fall below about 235° F.

The condensate that is pumped to an HRSG to be converted into saturated steam will typically arrive at the feedwater heater at about 100° F. But if directed through the feedwater heater at that temperature, sulfuric acid will condense on the downstream surfaces of the feedwater heater. To maintain all of the surfaces of the feedwater heater, which surfaces are typically coils, above the acid dew point temperature, in some HRSGs some of the low-temperature feedwater is diverted directly to the first evaporator at a bypass (FIG. 1). This reduces the load on the feedwater heater. Typically, some of the heated water that has passed through the coils of the feedwater heater, instead of flowing to the first evaporator, is re-circulated to mix with the cooler condensate, so that the temperature of the water entering the coils exceeds the acid dew point temperature. The bypass water, when employed, does not achieve the benefit of an initial temperature rise in the feedwater heater and decreases the efficiency of the HRSG. The re-circulation requires a re-circulation pump and valve and further requires bringing the feedwater that is discharged to a temperature higher than otherwise may be necessary. That too decreases efficiency. Moreover, the temperature of the exhaust gas as it passes into the feedwater heater is often not much greater than the temperature of the water leaving the feedwater heater, and as a consequence the feedwater heater must contain a rather large and expensive grouping of coils. The large feedwater heater coupled with the bypass results in a significant pressure drop across them, and this imposes a substantial load on the feedwater pump.

The conventional procedure for addressing the condensation of acid, that is to say a feedwater heater with re-circulation, and perhaps a bypass as well, works reasonably well where the exhaust gas derives from the combustion of natural gas, which has a relatively low acid dew point temperature on the order of 140° F. With a low acid dew point temperature, the feedwater heater can see a relatively large temperature differential between the exhaust gas as it flows through the feedwater heater and the feedwater in the coils of the heater, so the coils need not present an excessively large surface area. However, when dew point temperature is higher, such as at 230° F. for exhaust gas derived from some fuel oils, a large temperature differential is not available at the feedwater heater (it becomes very tight—FIG. 1A). As a consequence, the feedwater heater requires a large bundle of coils, making a conventional feedwater heater expensive in its own right and also requiring a significant head from the condensate pump just to force water through it and into the economizers and low pressure evaporator beyond it.

Apart from that, most HRSGs produce superheated steam at three pressure levels—low pressure (Ip), intermediate pressure (ip) and high pressure (hp). The feedwater heater typically discharges some of the heated feedwater directly into a low pressure evaporator, so the condensate pump must not only overcome the head required to force the feedwater through the feedwater heater, but also the pressure at which the Ip evaporator operates. The remainder of the feedwater goes to several ip and hp pumps which force it through an economizer to elevate its temperature still further so that it is better suited for ip and hp evaporators located still further upstream in the flow of exhaust gases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a feedwater heater and low pressure evaporator and economizer as used in HRSGs of the prior art;

FIG. 1A is a graphical representation of the temperature differential between the feedwater in and exhaust gas flowing through a feedwater heater and economizer of the prior art;

FIG. 2 is a schematic view showing a heat recovery steam generator having an improved feedwater heater, economizer, and pump arrangement, all constructed in accordance with and embodying the present invention;

FIG. 2A is a graphical representation of the temperature differential between the feedwater in and the exhaust gas flowing through the feedwater heater and economizer of the HRSG depicted FIG. 2;

FIG. 3 is a schematic view of an alternative heat recovery steam generator embodying the present invention; and

FIG. 3A is a graphical representation of the temperature differential between the feedwater in and exhaust gas flowing through the feedwater heater and economizer of the HRSG depicted in FIG. 3.

BEST MODES FOR CARRYING THE INVENTION

Referring now to the drawings, a heat recovery steam generator (HRSG) A (FIG. 2) that extracts heat from a hot gas flow provides superheated steam at several pressure levels. That steam may be directed to a steam turbine to power it. After passing through the turbine the steam discharges at a lower pressure and temperature and is condensed into subcooled liquid water which is circulated back to the HRSG A to again be converted into superheated steam.

The HRSG A includes (FIG. 2) a housing 2 that is basically a duct having an inlet 4 and an outlet 6. The HRSG A also includes a series of heat exchangers contained within the housing 2, and their functions are to a large measure described by their names. In addition, the HRSG A includes pumps, valves, and lines or conduits connecting the heat exchangers, pumps and valves together into the functioning HRSG A. Hot exhaust gas derived from the combustion of a fossil fuel enters the housing 2 at its inlet 4, passes through the several heat exchangers, which extract heat from it, and is discharged at the outlet 6. Typically, the exhaust gas represents the exhaust of a gas turbine that burns natural gas or fuel oil or even coal. It may enter the housing 2 at between 900° F. and 1200° F. The combustion produces primarily carbon dioxide and water. But fossil fuels invariably contain trace amounts of sulfur, so the combustion also produces small amounts of sulfur dioxide and sulfur trioxide. To prevent this combination from condensing into sulfuric acid on surfaces of the heat exchangers, those surfaces must be maintained above the acid dew point temperature for the exhaust gas. Sulfuric acid, of course, is highly corrosive and will attack most metals, including the metals from which the heat exchangers are formed. The water flows through the HRSG A generally in the direction opposite to the direction in which the hot exhaust gas flows, so the heat exchangers most vulnerable are those located at the back end, that is to say, the heat exchangers located downstream in the flow of the exhaust gas. This holds particularly true when the exhaust gas has a high acid dew point, such as the gas derived from the combustion of fuel oil. Surfaces that this gas encounters should be maintained above at least 230°.

Actually, the heat exchangers that are most vulnerable are those that heat the incoming water, called feedwater, so that the subcooled feedwater water is more efficiently converted into steam. Notwithstanding the relatively high temperatures at which these heat exchangers are capable of operating, they maintain very effective temperature differentials between the liquid water in them and the hot exhaust gas passing over them. This is achieved by dividing the heating of the feedwater into two components—an evaporative component and a sensible component. In the evaporative component the temperature of the feedwater remains constant. In the sensible component the temperature of the feedwater rises from the constant temperature in the evaporative component.

Beginning at the back end of the HRSG A, an initial or condensate pump 10 pumps liquid water—typically subcooled condensate from a steam turbine—into a feedwater line 12 that leads to an evaporative feedwater heater 14 where the evaporative component of the heating occurs. In contrast to a feedwater heater in a conventional HRSG, which includes a large bank of coils and a re-circulation pump, the feedwater heater 14 has a steam drum 16 and a small bank of coils 18 below the steam drum 16. The steam drum 16 is connected to the lower ends of coils 18 through a downcomer 20 that enables liquid water to flow from the steam drum 16 into the lower ends of the coils 18. The upper ends of the coils communicate with the lower region of the steam drum 16 through risers 22. Within the coils 18 some of the water transforms into saturated steam, and the coils 18 discharge liquid water and saturated steam into the steam drum 16 through the risers 22. The water circulates by natural convection through the drum 16, the downcomer 20, the coils 18 and the risers 22 at a constant temperature, which is the saturation temperature for the steam and water, although a pump may be provided to assist the natural convection.

The steam drum 16 contains saturated liquid water and above the liquid water saturated steam. The liquid water must exist at the minimum temperature for the coils 18 to which it is directed through the downcomer 20, and that temperature is usually slightly above the acid dew point temperature. When fuel oil produces the exhaust gas, that the dew point temperature is often 230° F. but could be higher. The condensate will enter the steam drum 16 at a relatively low temperature, typically about 100° F., and there it condenses the saturated steam and mixes with the saturated water in the steam drum 16. Of course, it undergoes a rise in temperature—indeed, instantaneously to the minimum temperature of the water discharged into the downcomer 20 and thence to the coils 18. The hot exhaust gas passing through the coils 18 elevates the energy of the liquid water in the coils 18 and converts some of it to saturated steam, much like a circulation-type evaporator. The pressure of the steam and liquid water in the steam drum 16 controls the temperature of that saturated water and steam.

Some of the liquid water in the steam drum 16 re-circulates through the feedwater heater 14. However, the volume of the water in the steam drum 16—remains generally constant, so while some water re-circulates, more water is displaced by the incoming feedwater introduced through the feedwater line 12. The displaced water flows out of the drum 16 through a discharge line 24 that leads to three feedwater pumps—a low pressure (Ip) pump 26, an intermediate pressure (ip) pump 28, and a high pressure (hp) pump 30. Actually, two or even all three of the pumps 26, 28, 30 may be combined into a single pump having multiple stages or discharges. All three of the pumps 26, 28, 30 discharge liquid water at nearly the same temperature, which is essentially the temperature of the saturated water in the steam drum 16 of the feedwater heater 14, although they discharge at progressively higher pressures and thus the water again becomes subcooled.

The Ip pump 26 delivers liquid water through a low pressure supply line 32 to a low pressure (Ip) evaporator 34 which, operating on the natural circulation principle, converts that water into saturated team. To this end, the Ip evaporator 34 has a steam drum 36 into which the supply line 32 opens. In addition, it has coils 38 located below the drum 36, a downcomer 40 leading from the drum 36 to the lower ends of the coils 38, and risers 42 leading from the upper ends of the coils 38 to the steam drum 36. The Ip evaporator 34 produces saturated steam at a pressure and temperature greater than the pressure and temperature at which the feedwater heater 14 operates. The saturated steam A leaves the Ip evaporator 34 through a discharge line 44 that extends away from the top of the steam drum 36.

In the operation of the Ip evaporator 34 subcooled liquid water enters the steam drum 36 through the Ip supply line 32. There the water mixes with saturated steam—it being the product of the Ip evaporator 34 as a consequence of liquid water entering the downcomer 40 and flowing to the bottom of the coils 38. Upon rising through the coils 38 some of the water converts to saturated steam that enters the steam drum 36 along with the water that remains in the liquid phase. The temperature of both exceeds the temperature of the water entering the steam drum 36 at the supply line 32 from the Ip pump 26, and that temperature is essentially the temperature of the water discharged by the feedwater heater 14. The saturated steam leaves the steam drum 36 through the discharge line 44, while the liquid water re-circulates by natural convection through the downcomer 40.

The discharge line 44 directs the saturated steam to a low pressure (Ip) superheater 50 located upstream in the flow of exhaust gas from the feedwater heater 14 and the evaporator 34. It converts the saturated steam into superheated steam. The superheated steam leaves the Ip superheater 50 through an Ip steam line 52 that may lead to the low pressure stage of a steam turbine. The steam line 52 controls the pressure at which the Ip evaporator 34 operates.

The discharge line 44 leading away from the steam drum 36 of the Ip evaporator 34 also connects with the steam drum 16 of the feedwater heater 14 through a pegging line 54 and pegging valve 56 in the line 54. Owing to the Ip pump 26 interposed between the feedwater heater 14 and the Ip evaporator 34, the pressure in the steam drum 36 of the latter exceeds the pressure in the steam drum 16 of the former. The pegging valve 56 admits the higher pressure steam from the Ip evaporator 34 into the steam drum 16 of the feedwater heater 14 such that the feedwater heater 14 operates at a desired pressure. And that pressure correlates with a saturation temperature that exceeds the dew point temperature of the exhaust gas as it flows through the coils 18 of the evaporator—indeed, as it flows through the last or downstream of the coils 18. A pressure sensor monitors the pressure of the water in the steam drum 16 of the feedwater heater 14 and produces a signal to which the pegging valve 56 responds—opening and closing so that the saturation temperature in the steam drum 16 remains at the desired level. The temperature in the steam drum 16 should of course be above the acid dew point temperature, but should not exceed it by more than about 15° F. and preferably by no more than about 5° F. to achieve maximum efficiency in the HRSG A.

The ip pump 28 and the hp pump 30 deliver water heated by the feedwater heater 14, but now subcooled owing to the increase in pressure, to an economizer 64 which heats both the ip and hp water in separate coils to still higher temperatures, yet the ip and hp water remains in the liquid phase. The economizer 64 provides the sensible component of the heating. Preferably the coils for the ip and hp steam are located side by side in the duct 2 so that each encounter exhaust gas at the same temperature, although they may be located one ahead of the other. The economizer 64 discharges the liquid water into two discharge lines—an ip discharge line 66 and an hp discharge line 68.

The ip discharge line 66 leads to a combined unit 70 that serves as an ip evaporator and an initial ip superheater, it being located upstream in the flow of exhaust gas from the economizer 64. The ip combined unit 70, by extracting heat from the exhaust gas, produces saturated steam and later superheated steam that leaves through an ip line 72. The hp discharge line 68 leads to another hp economizer 74 located upstream from the ip combined unit 70. The economizer 74 heats the liquid water still hotter and discharges it through an hp line 76.

Both the ip line 72 and the hp line 76 connect with and feed a combined hp evaporator and ip superheater unit 78, there being in the unit 78 separate coils for the hp water-steam and ip superheated steam. The unit 78 discharges superheated steam at an intermediate pressure through an ip steam line 80 that may lead to a steam turbine. The unit 78 also delivers saturated steam to an hp connecting line 82. It leads to an hp superheater 84 that converts the saturated steam into superheated steam. The superheater 84 discharges the superheated steam through an hp steam line 86 that may lead to the steam turbine.

In the operation of the HRSG A, hot exhaust gas enters the housing 2 at its inlet 4 and flows continuously through it to the outlet 6, at which it may be discharged to the atmosphere. In so doing the gas encounters the hp superheater 84, the hp evaporator and ip superheater unit 80, the hp economizer 74, the combined ip evaporator and superheater unit 70, the Ip superheater 50, the hp and ip economizer 64, the Ip evaporator 34, and the feedwater heater 14 in that order. Feedwater, which may be a condensate at a temperature of 100° F. or lower, enters the HRSG A at the condensate pump 10 which forces it through the feedwater line 12 into the steam drum 16 of the feedwater heater 14 where it undergoes an instantaneous rise in temperature (FIG. 2A). Actually, the head overcome by the pump 10 is minimal and could be 5 psi or lower, inasmuch as the feedwater heater 14 operates at a low pressure and relies on convection to circulate water—and steam—through it. The drum 16 already contains some liquid water as do the coils 18 below the drum 16. Moreover, the drum 16 contains saturated steam above the water in it, and the pressure at which that steam exists determines the temperature of the liquid water, which is the saturation temperature of the steam and water at that pressure. That temperature, which is constant, should exceed the acid dew point temperature of the exhaust gas, but preferably not by more than about 5° F. The liquid water in the steam drum 16 flows through the downcomer 20 to the lower ends of the coils 18 through which it rises. The heat extracted from the exhaust gas as it flows over the coils 18 converts some of the liquid water into saturated steam which rises through the coils 18 and risers 22 along with the remaining water. Both enter the steam drum 16. As with a natural circulation evaporator, the water circulates through the feedwater heater 14 by convection, natural or forced, and it remains at a constant temperature. The incoming feedwater introduced into the steam drum 16 through the feedwater line 12, displaces water from the drum 16, and that displaced water flows through the discharge line 24 to the three pumps 26, 28, 30 at an elevated temperature that exceeds the dew point temperature of the exhaust gas, but at the low pressure at which the feedwater heater 14 operates. Apart from that, the incoming feedwater, while undergoing an instantaneous rise in temperature, condenses the saturated steam within the steam drum 16, thus preventing an excess of steam in the drum 16. In any event, steam does not escape from the drum 16; it simply reverts to saturated liquid water. The feedwater heater 14 provides the evaporative component for heating the feedwater.

The Ip pump 26 delivers the water at a higher pressure, so it is again subcooled, through the Ip supply line 32 to the steam drum 36 of the Ip evaporator 34 where it mixes with higher temperature water already in the steam drum 36, water which is located below saturated steam that also occupies the steam drum 36. The water leaves the steam drum 36 through the downcomer 40 which directs it into the lower ends of the coils 38. Being subjected to the exhaust gas at a higher temperature than the coils 18 of the feedwater heater 14, the coils 38 of the Ip evaporator 34 convert some of the higher pressure water into saturated steam which along with the remainder of the water flows upwardly into the steam drum 36, with the steam occupying the upper regions of the drum 36. Owing to the higher pressure in the evaporator 34, the saturated steam and water in its steam drum 36 exist at a higher temperature than the water and steam in the feedwater heater 14. Again the circulation is by natural convection, although the evaporator 34 may have a pump assist. From the steam drum 36 the saturated steam flows through the discharge line 44 to the Ip superheater 50. Being located upstream from the Ip evaporator 34 in the flow of exhaust gas, the Ip superheater 50 sees temperatures higher than the Ip evaporator 34—indeed, temperatures high enough to convert the saturated steam into superheated steam. That steam escapes through the Ip steam line 52.

Some of the Ip saturated steam that leaves steam drum 36 of the Ip evaporator 34 serves as pegging steam for controlling the pressure and temperature of the saturated steam in the steam drum 16 of feedwater heater 14 and likewise the temperature of the liquid water in the steam drum 16—and that is the water that circulates through the downcomer 20 and into the coils 18 of the feedwater heater 14

The higher pressure pegging steam flows through the pegging line 54, it being discharged into the steam drum 16 of the feedwater heater 14 at a pressure determined by the pegging valve 56. That pressure is such that the saturation temperature of the steam and water in the steam drum 16 is slightly above the acid dew point temperature of the exhaust gas at the feedwater heater 14.

The ip pump 28 delivers water that has been heated at the feedwater heater 14, discharging it at an intermediate pressure higher than the pressure produced by the Ip pump. That subcooled water flows through the combined economizer 64 where it is elevated to a higher temperature. It then flows to the combined ip evaporator and ip superheater unit 70 from which it leaves as superheated steam. That superheated steam undergoes a further elevation in temperature at the ip coils of the combined hp evaporator and ip superheater unit 78, from which it is discharged into the ip steam line 86. The economizer 64 provides the sensible component for heating the feedwater.

The hp pump 30 also delivers water heated at the feedwater heater 14, it being directed in a subcooled condition into the combined economizer 64 where its temperature is elevated, and thence into the hp coils of the combined hp evaporator and ip superheater unit 78 where it is converted to saturated steam. The saturated steam flows into the hp superheater 84 where it becomes superheated steam. That steam leaves through the hp steam line 86.

The evaporative feedwater heater 14 with its evaporative component combined with the economizer 64 with its sensible component produces wider temperature differentials between the exhaust gas flowing over those heat exchangers and the liquid water flowing through them than could be achieved by a conventional feedwater heater, even one operating with an upstream economizer. In this regard, the temperature of the water in the feedwater heater 14 remains constant and does not rise with the higher temperatures at its end upstream in the flow of exhaust gas. As a consequence, a relatively large temperature differential exists at the upstream end and further downstream as well (FIG. 2A). Moreover, the water flows on to the economizer 64 at this lower temperature, so the coils at the downstream end of the economizer 64, reference being to the flow of the exhaust gas, also see a significant temperature differential between the gas following over those coils and the feedwater in them.

By way of example—and example only—exhaust gas derived from the combustion of fuel oil and having an acid dew point temperature of 230° F., approaches the combined economizer 64 in the housing 2 at 600° F. The economizer 64 extracts enough heat to lower the temperature to 400° F., and at that temperature it flows into the Ip evaporator 34 which extracts more heat, causing the exhaust gas to flow on to the feedwater heater 14 at 300° F. The feedwater heater 14 extracts still more heat from the exhaust gas, enough to reduce its temperature to 245° F., and at that temperature the exhaust gas leaves the housing 2 through the outlet 4. The condensate pump 10 delivers 500,000 lb/hr of feedwater to the steam drum 16 of the evaporator 14 at 100° F., and this requires only enough head to overcome whatever pressure the Ip evaporator 34 produces in the interior of the drum 16 which may be as low as 8 psig. Indeed, that pressure is sufficient to keep the saturation temperature of the steam and water in the steam drum 16 at 235° F. And the saturated water leaves the drum 16 at that temperature and enters the coils 18 where it remains at 235° F. while some of it is converted to steam. Since the temperature of the water in the coils 18 does not rise, a substantial temperature differential exists between the water and exhaust gas. Being at a temperature above the acid dew point temperature of the exhaust gas, the coils 18 do not experience condensation of acid on their surfaces. Saturated water also leaves the steam drum 16 through the discharge line 24 at 235° F., flowing into the pumps 26, 28, 30 at that temperature. The Ip pump 26 directs 100,000 lb/hr to the steam drum 36 of the Ip evaporator 34 at 235° F. The Ip evaporator 34 converts that water to saturated steam, discharging 100,000 lb/hr. through the discharge line 44 at 350° F. The ip pump 28 and hp pump 30 deliver the remaining 400,000 lb/hr through the supply lines 60 and 62, again at 235° F., to the combined economizer 64 which extracts more heat—indeed quite efficiently because the liquid water enters the economizer 64 at about 235° F., producing a large temperature differential between the exhaust gas and feedwater at the downstream end of the economizer and farther upstream as well.

The HRSG A equipped with the feedwater heater 14 and companion economizer 64 heats a large volume of feedwater without requiring the re-circulation and bypass utilized by a feedwater heater for a conventional HRSG. Moreover, the feedwater heater 14 heats the feedwater with a relatively small bank of coils 18, since the water in the coils 18 remains at the saturation temperature, and a relatively large temperature differential exists between the water in those coils 18 and the exhaust gas flowing over them. The water circulates through the coils 18 by natural or forced convection, so the condensate pump 10 need not overcome whatever resistance the coils 18 may otherwise impose on the flow. Since the coils 18 see a substantial drop in the temperature of the exhaust gas as it flows across them, they are highly efficient. Upon leaving the feedwater heater 14, the liquid water at the relatively low saturation temperature in the heater 14 flows on to the economizer 64 through the ip supply line 60 and hp supply line 62, so that at the downstream end of the economizer 64, another substantial temperature differential exists.

In contrast, the feedwater heater of a conventional HRSG will normally have a large bank of coils designed to extract heat over a relatively small temperature differential in the flow of exhaust gas, and those coils offer significant resistance to the flow of feedwater. The condensate pump must overcome that resistance. Indeed, the feedwater heater 14, owing to the absence of re-circulation in the heater 14 itself, need not heat the feedwater to as high a temperature as a conventional feedwater heater. Moreover, the temperature differential between the feedwater at the coils and upstream economizer of a conventional feedwater heater is not as great as seen by the feedwater heater 14 and economizer 64 (compare FIG. 1A and FIG. 2A).

The components operating under the pressures developed by the ip pump 28 and the hp pump 30 may vary from that described in the foregoing text and depicted in the drawings. Moreover, one of the pumps 28 or 30 and the components it services may be eliminated altogether, producing an HRSG that delivers superheated steam at two pressures. Also, the pegging steam supplied to the drum 16 of the feedwater 14 may come from a source of pressurized steam other than the drum 36 of the Ip evaporator 34. While the evaporator 34 and the evaporative components of the combined units 70 and 80 are depicted and described as natural circulation evaporators, they may take the form of pump-assisted circulation evaporators or even once-through evaporators.

An alternative HRSG B (FIG. 3) closely resembles the HRSG A and as such includes the same feedwater heater 14 and Ip evaporator 34. However, the three pumps 26. 28, 30 feed another combined economizer 90 located between the feedwater heater 14 and the Ip evaporator 34 in the flow of the exhaust gas As such liquid water enters the economizer 90 at the saturation temperature of the water discharged by the feedwater heater, so a substantial temperature differential exists between the water in the combined economizer 90 and the exhaust gas flowing through the economizer 90 (FIG. 3A). This likewise contrasts with the heating of feedwater in a conventional HRSG (compare FIG. 1A and FIG. 3A). The economizer 90 thus requires a relatively small bundle of coils for each of the Ip, ip and hp water. Also, the feedwater heater 14 may include a deaerator 92 into which the feedwater line 12 and pegging line 54 open, so that the feedwater and pegging steam flow into the deaerator 92. It in turn communicates with the steam drum 16 through a connecting line 94. 

1. In an HRSG for extracting heat from an exhaust gas that flows through the HRSG and has an acid dew point temperature, and for utilizing that heat to convert subcooled feedwater into steam, the improvement comprising: a feedwater heater located in the flow of the exhaust gas, the feedwater heater converting subcooled feedwater at a low temperature into saturated steam and saturated water, with the temperature of the saturated water being above the acid dew point temperature of the exhaust gas; a feedwater pump receiving water from the feedwater heater and elevating the pressure of the water; and an economizer located in the flow of the exhaust gas upstream from the feedwater heater and connected to the feedwater pump for receiving feedwater at an elevated pressure from the pump and elevating the temperature of that feedwater.
 2. The combination according to claim 1 wherein the feed water heater comprises: a steam drum into which the subcooled feedwater is directed; a coil located below the steam drum within the flow of the exhaust gas and at its lower and upper ends communicating with the steam drum such that water from the steam drum circulates through coil where some of the water is converted to saturated steam; and a feedwater discharge line connecting the steam drum and the feedwater pump.
 3. The combination according to claim 2 wherein the steam drum is connected to a source of pegging steam for controlling the pressure in the steam drum.
 4. The combination according to claim 2 and further comprising: an initial pump that receives subcooled water at a low pressure and temperature; and a feedwater line connecting the initial pump and the drum of the feedwater heater for directing feedwater from the initial pump into the drum.
 5. The combination according to claim 4 and further comprising an evaporator including a coil located in the flow of exhaust gas upstream from the coil of the feedwater heater and receiving heated feedwater at an elevated pressure produced by the feedwater pump and producing saturated steam; and a steam discharge line through which saturated the steam escapes.
 6. The combination according to claim 5 wherein the evaporator further includes a steam drum with which the coil of the evaporator at its upper and lower ends communicates, so that water from the steam drum circulates through the coil and some of it transforms into saturated steam that flows into the steam drum.
 7. The combination according to claim 6 and further comprising: a pegging line that opens into the steam drum of the feedwater heater and communicates with the steam drum of the evaporator; and a pegging valve in the pegging line for controlling the pressure of steam admitted to the steam drum of the feedwater heater through the pegging line.
 8. The combination according to claim 5 and further comprising a superheater located in the flow of the exhaust gas upstream from the coil of the evaporator and connected to the discharge line of the evaporator for converting saturated steam received from the evaporator into superheated steam.
 9. The combination according to claim 5 wherein the coil of the evaporator is located between the economizer and the coil of the feedwater heater in the flow of the exhaust gas.
 10. The combination according to claim 5 wherein the economizer is located between the coil of the evaporator and the coil of the feedwater heater in the flow of the exhaust gas, and the feedwater pump forces feedwater from the feedwater heater through the economizer and into the evaporator.
 11. The combination according to claim 5 and further comprising another evaporator located in the flow of the exhaust gas upstream from the economizer for receiving heated feedwater from the economizer and converting it into saturated steam.
 12. The combination according to claim 1 wherein the feedwater heater produces saturated water, the temperature of which exceeds the acid dew point temperature by no more than about 15° F.
 13. A process for furnishing liquid water at an elevated temperature and pressure to an evaporator located in a flow of exhaust gas having an acid dew point temperature, said process comprising: extracting heat from the flow of exhaust gas to heat subcooled feedwater to a saturation temperature above the acid dew point temperature so as to provide saturated feedwater; elevating the pressure of the saturated feedwater to produce higher pressure feedwater; and upstream from the extraction of heat to create saturated feedwater, extracting more heat from the flow of exhaust gas to heat the higher pressure feedwater to a higher temperature.
 14. The process according to claim 13 and further comprising upstream from the extraction of heat to heat the higher pressure feedwater extracting more heat from the flow of the exhaust gas to convert the higher pressure feedwater into saturated steam.
 15. The process according to claim 13 wherein extracting heat from the exhaust gas to heat subcooled water comprises: introducing the subcooled water into a steam drum; and circulating the water from the steam drum through a coil located in the flow of the exhaust gas such that some of the water converts into saturated steam and both saturated steam and saturated water flow into and occupy the steam drum.
 16. The process according to claim 15 and further comprising controlling the temperature of the saturated steam and water in the feedwater heater by controlling the pressure of the steam in the steam drum.
 17. The process according to claim 15 and further comprising: converting the higher pressure feedwater into higher pressure saturated steam; and controlling the temperature of the saturated water in the steam drum by subjecting it to the higher pressure saturated steam.
 18. The process according to claim 15 wherein the temperature of the saturated steam and water in the steam drum does not exceed the acid dew point temperature by more than about 15° F. 